The present invention relates to a code modulated communication system, and more particularly to a system and method for acquiring synchronization in such a code modulated communication system.
In a code modulated communication system such as a direct sequence spread spectrum (SS) system, the transmitter multiplies the data by a periodic pseudo-random (PN) code with a chip rate which is much higher than the data rate. This process spreads the frequency bandwidth of the transmitted signal. PN code bits are called chips to differentiate them from data bits. The transmitted signal is received at a receiver, and the transmitted signal is despread at the receiver to its original bandwidth using a replica of the spreading code used at the transmitter. Before being able to despread the signal and extract the data, the receiver must synchronize its locally generated PN code with the PN code of the received signal. To facilitate synchronization, the transmitter typically also transmits the PN code, or a related PN code, alone, without data modulation. The receiver synchronizes its local PN code with the transmitted PN code prior to data detection.
During SS PN acquisition the received and local codes are synchronized to within a coarse accuracy of plus or minus one half of the duration of one PN chip. It is also known to use tracking to provide fine synchronization.
There exists a number of techniques for SS PN code acquisition. For further information regarding SS PN code acquisition, the reader is referred to chapter one of Spread Spectrum Communications, Vol. III., by M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levitt, Rockville, Md.: Computer Science Press, 1985, the disclosure of which is hereby incorporated by reference.
In existing acquisition applications, a threshold is set at a level to prevent noise samples from causing false indication. In a search mode, samples of received signals are correlated with a local code which is a replica of the code of the transmitted signal using an I-Q square law envelope detector (SLED) having a correlator in which the local code is loaded. Correlation values are outputted from the SLED, and when a correlation value exceeds a pre-selected threshold, the value and its corresponding index are recorded. If a tested correlation value does not exceed the threshold, the next index is tested. When a correlation value exceeds the threshold, a verification mode is performed to determine if the corresponding index represents the existence of synchronization. At a low signal to noise ratio, or during a channel fading period, the sample corresponding to the correct match will not necessarily exceed the threshold. Consequently, in some cases multiple code periods have to be tested in the search mode before entering the verification mode. Disadvantageously, this results in delay in acquisition, verification and decoded data reception.
Prior to each new search period, all information concerning previous indexes from previous searches is cleared from memory.
With known verification techniques, only one index i is abstracted during the search mode and tested during the verification mode. Disadvantageously, restricting verification to only one candidate index results in a high probability of missing the index corresponding to synchronization and resultant need for time consuming repetition of the search.
Accordingly a need exists for a code modulated communication system with an improved and faster synchronizing acquisition system and method which utilizes the history of comparing samples and associated indexes of received transmitted signals with a local code, especially when operating in a fading channel and/or low signal-to-noise ratio (SNR) environment.